The invention generally relates to a phase locked loop.
A phase locked loop (PLL) is used for purposes of synchronizing the phases of two signals together. For example, FIG. 1 depicts a typical PLL 5 that includes a Voltage Controlled Oscillator (VCO) 7, a loop filter 9 and a phase detector 10. The phase detector 10 includes an input terminal 13 that receives an input signal that is “locked” onto by the PLL 5. In this manner, the phase detector 10 compares the input signal with an output signal (of the PLL 5) that is generated by the VCO 7 at its output terminal 11. Based on the detected phase difference between the input and output signals, the phase detector 10 generates a control signal that propagates through the loop filter 9 to the input terminal 8 of the VCO 7. The VCO 7 controls the frequency of the output signal based on the voltage level of the control signal. Due to this closed loop control, the PLL 5 “locks” onto the phase of the input signal so that the output signal has a predefined phase relationship (a zero, ninety or one hundred eighty degree relationship, as examples) with respect to the input signal.
The VCO 7, in its steady state, typically operates at a frequency that is either the same or an integer multiple of the frequency of the input signal. Thus, typically, the output signal has a frequency that is the same as or an integer multiple of the input signal. However, such an arrangement may be subject to noise. In this manner, for a typical oscillator, such as an inductor capacitor (LC)-based tank circuit, the oscillator output noise spectrum (i.e., the noise that is present in the oscillator's output signal) may be defined by the following equation:                                                                                       H                ⁡                                  (                                      δ                    ⁢                                                                                   ⁢                    ω                                    )                                                                    2                    ≈                                    1                              4                ⁢                                                      π                    2                                    ·                  Q                                                      ⁢                                          (                                                      ω                    O                                                        δ                    ⁢                                                                                   ⁢                    ω                                                  )                            2                                      ,                            Eq        .                                   ⁢        1            where “|H(δω)|2” represents the output noise spectrum of the oscillator, “Q” represents the Q factor of the inductor, “ωo ” represents the resonant frequency of the oscillator and “δω” represents the spectral frequency.
As depicted in FIG. 2 in a graph of the oscillator's output noise versus frequency, the oscillator is highly susceptible to external noise at the resonant frequency, the fundamental frequency of operation of the oscillator. This external noise may be introduced by, for example, the substrate in which the oscillator is fabricated and may also be attributable to the power supply that powers the oscillator. Furthermore, the extent of the introduced external noise is specifically dependent upon the integrated circuit fabrication technology. For example, in complementary metal-oxide-semiconductor (CMOS) fabrication, the noise may be attributable to substrate coupling, which modulates the threshold voltage of the metal oxide semiconductor field-effect-transistors (MOSFETs) of the oscillator, and the noise may also be attributable to, for example, capacitive coupling effects present at the source and drain terminals of the MOSFETs of the oscillator.
Regardless of the sources of the noise, in a typical PLL, the presence of noise in the output signal from the oscillator introduces a phase noise, or jitter, between the input and output signals of the PLL, thereby adversely affecting operation of the PLL.
Thus, there is a continuing need for an arrangement to address one or more of the problems that are stated above.